1. Field of the Invention
The present invention relates to a power supply device that outputs multiple power supplies with different voltages, a power supply device control method, and an image forming apparatus.
2. Description of the Related Art
There are many conventional electronic devices that need multiple power supplies of different voltages. For example, an inkjet printer that ejects ink from a printer head and performs printing requires multiple power supplies of different voltages including a power supply with a first voltage to drive operational units including a display device, such as a liquid crystal display, and a motor; a power supply with a second voltage to drive the printer head; and a power supply with a third voltage to drive a logical circuit.
With such multiple power supplies at different voltages, there may be a limitation on the order in which the power is supplied. In this example, a case will be considered where a power supply device supplies a head driver integrated circuit (IC) to drive the printer head with a first power supply for the head driver IC and a second power supply, to drive the printer head, with a voltage much higher than that of the first power supply. In this case, if the supply of the second power supply to the head driver IC is continued even after the output of the first power supply to the head driver IC is stopped, the through current due to the second power supply flows through the head driver IC via the parasitic diode in the head driver IC, which may damage the head driver IC.
For this reason, conventional power supply devices are known that have a function of controlling when multiple power output circuits of different output voltages start or stop their outputs. For example, Japanese Patent No. 4279235 discloses a configuration that monitors the power supplied from an external power supply device by using a photocoupler and that controls when the power output circuit is stopped by using transistor switching and by using the length of time for which the capacitor discharges. The configuration according to Japanese Patent No. 4279235 can control, even if the supply of power from the external power supply device is cut out, when the multiple power output circuits stop their outputs such that they are stopped in a predetermined order.
Conventional power supply devices control when the output stops by using the time period for which the capacitor discharges in the power supply device and thus the power output of the power supply device ensures when the output stops. The capacitor connected to the circuit of a device supplied with power, such as a printer head driver circuit, is not taken into consideration. Thus, for example, when a capacitor with a large capacitance is connected to the printer head driver circuit, to which a power supply is supplied, to rectify the waveform, a problem occurs in that when the power supply device stops the output and when the output of the voltage of the device supplied with power is stopped might be different.
As an example, a case will be considered where a large-capacitance capacitor is connected to the above-described second power supply in the power-supplied device. In this case, even if the power supply device stops the first and second power outputs simultaneously, the supply of the second power supply is continued also after the supply of the first power supply is stopped due to the operation of the capacitor. Accordingly, there is a risk that the head driver IC might be damaged.
The above-described configuration of Japanese Patent No. 4279235 cannot solve the problem that, when a large-capacitance capacitor is connected to the circuit of the power-supplied device, the time point when the output of the power supply device stops and the time point when the power supply to the power-supplied device stops may be different from each other.
In order to avoid such a problem, a capacitor for the power supply device may be set in consideration of the capacitance of the capacitor connected to the printer head drive circuit. However, this case has a problem in that design freedom is narrowed.
The method of controlling when the output of the power supply is stopped by using the charging or discharging of the capacitor has a problem in that it is difficult, when an irregular operation is performed, e.g., when the power supply is turned off during charging, to cause a given time difference between the time points when the output of power supplies at different voltages stops.
As described above, conventional power supply devices have a problem that, when a power supply device is stopped, a power-supplied device cannot follow a power supply sequence specific to the device supplied with power, which might cause instability in the device operations or damage the device.
There is a need, when multiple power supplies of different voltages are supplied, to prevent unnecessary high-voltage supplies to a device being supplied with power when the power supply is stopped.